Population Structure of Listeria monocytogenes Serotype 4b Isolates from Sporadic Human Listeriosis in the United States, 2003-2008

Authors: LEE, SANGMI - North Carolina State University; Ward, Todd; GRAVES, LEWIS - Centers For Disease Control And Prevention (CDC) - United States; TARR, CHERYL - Centers For Disease Control And Prevention (CDC) - United States; SILETZKY, ROBIN - North Carolina State University; KATHARIOU, SOPHIA - North Carolina State University

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2014
Publication Date: 6/1/2014
Publication URL: http://handle.nal.usda.gov/10113/60356
Citation: Lee, S., Ward, T.J., Graves, L.M., Tarr, C., Siletzky, R.M., Kathariou, S. 2014. Population structure of Listeria monocytogenes serotype 4b isolates from sporadic human Listeriosis in the United States, 2003-2008. Applied and Environmental Microbiology. 80(12):3632-3644.

Interpretive Summary: Listeria monocytogenes is a food-borne bacterium that can cause serious illness in humans and animals (listeriosis). Certain strains of this pathogen, which are known as serotype 4b strains, are disproportionately responsible for major outbreaks of listeriosis as well as sporadic illness. While previous analyses have identified three groups of closely related strains (EC1, ECII, and EC1a) that have repeatedly caused major outbreaks of listeriosis, relatively little is known about the contribution of different serotype 4b strains to sporadic listerisosis, even though most listeriosis is the result of sporadic illness. In this study, we characterized 136 serotype 4b isolates from sporadic listerisosis cases that occurred in the United States between 2003 and 2008. Our results demonstrate that most (56%) of the sporadic isolates belong to the epidemic clonal groups (EC1, EC2, and EC1a). However, 30-68% of the isolates in any given year were not members of the epidemic clonal groups, indicating that a very large portion of human illness is due to serotype 4b strains that have been studied much less thoroughly than the epidemic clone types. These results demonstrate that a diverse set of serotype 4b strains are responsible for sporadic illness in the United States and will be of direct interest to epidemiologists, public health and food inspection agencies, and scientists involved in food safety research.

Technical Abstract: Listeria monocytogenes can cause severe foodborne disease (listeriosis). Serotype 4b strains have resulted in numerous outbreaks, repeatedly involving three epidemic clones (ECI, ECII, and ECIa). Little is known about population structure of L. monocytogenes serotype 4b from sporadic listeriosis, even though most listeriosis cases are sporadic. Here we analyzed 136 serotype 4b isolates from sporadic cases in the United States between the years 2003-2008. ECI, ECII, and ECIa accounted for 32, 17, and 7%, respectively, of the isolates. Even though outbreak-associated clonal groups were major contributors to sporadic illness, annually 30-68% of isolates were outside of these ECs and several novel clonal groups were identified based on multilocus genotyping (MLGT) and hybridization profiles with a panel of DNA probes. Resistance to benzalkonium chloride was uncommon (3%) while 33 and 17% of the isolates were resistant to cadmium and arsenic, respectively. Resistance to these heavy metals was primarily found among EC isolates. Our analysis suggests a complex population structure of serotype 4b L. monocytogenes from sporadic disease, with important contributions by outbreak-associated strain types and several novel clonal groups; continuous monitoring will be warranted to assess longer term trends in clonality patterns and resistance attributes (disinfectant and heavy metals) in serotype 4b strains from sporadic listeriosis.